Fabrication method and fabrication apparatus for porous glass base material for optical fiber

ABSTRACT

According to a fabrication method for fabricating a porous glass base material for optical fiber, the orientation of a clad forming burner used to form the outermost layer of a clad-corresponding portion is changed further upward while glass fine particles are deposited during the period between a first timing and a second timing. At the first timing, the outer diameter of the porous glass base material for optical fiber has not reached a target outer diameter. The second timing is later than the first timing, and either a timing at which the outer diameter of the porous glass base material for optical fiber reaches the target outer diameter for the first time, or a timing prior to this timing.

The contents of the following Japanese patent application areincorporated herein by reference: No. 2017-199690 filed on Oct. 13,2017.

BACKGROUND 1. Technical Field

The present invention relates to a fabrication method and a fabricationapparatus for a porous glass base material for optical fiber.

2. Related Art

According to the Vapor-phase Axial Deposition (VAD) method, a pluralityof synthesizing burners are used to concurrently form acore-corresponding portion and a clad-corresponding portion of a porousglass base material for optical fiber (see Patent Document 1). PatentDocument 1: Japanese Patent No. 5697165

According to the fabrication method disclosed in Patent Document 1, theporous glass base material may crack during the initial stage of thedeposition of the glass fine particles.

SUMMARY

A first aspect of the present invention provides a fabrication methodfor fabricating a porous glass base material for optical fiber, in whicha core-corresponding portion corresponding to a core of optical fiber isformed by depositing glass fine particles onto a hanging seed rod, andat least a portion of a clad-corresponding portion corresponding to aclad of the optical fiber is formed by depositing glass fine particlesonto the core-corresponding portion. Here, the fabrication methodincludes a period during which, while glass fine particles are beingdeposited, a gradient of a clad forming burner used to form an outermostlayer of the clad-corresponding portion is changed toward apredetermined gradient relative to the porous glass base material foroptical fiber from a downward gradient compared with the predeterminedgradient.

A second aspect of the present invention provides a fabricationapparatus for fabricating a porous glass base material for opticalfiber, including a reaction vessel configured to house therein a hangingseed rod, a core forming burner configured to deposit glass fineparticles onto the seed rod to form a core-corresponding portion that isto be formed into a core of optical fiber, a clad forming burnerconfigured to deposit glass fine particles that are to be formed into aclad of the optical fiber, onto the core-corresponding portion to format least an outermost layer of a clad-corresponding portion that is tobe formed into the clad of the optical fiber, a driver configured tochange an orientation of the clad forming burner, and a controllerconfigured to control the driver to, while glass fine particles aredeposited, change a gradient of the clad forming burner used to form atleast the outermost layer of the clad-corresponding portion toward apredetermined gradient relative to the porous glass base material foroptical fiber from a downward gradient compared with the predeterminedgradient.

The summary clause does not necessarily describe all necessary featuresof the embodiments of the present invention. The present invention mayalso be a sub-combination of the features described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the structure and initial state of afabrication apparatus 10.

FIG. 2 schematically shows the state of the fabrication apparatus 10that can be observed after the deposition starts.

FIG. 3 schematically shows a different state of the fabricationapparatus 10 that can be observed after deposition starts.

FIG. 4 shows the ratio of an effective portion 82 to a porous glass basematerial 30.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, some embodiments of the present invention will bedescribed. The embodiments do not limit the invention according to theclaims, and all the combinations of the features described in theembodiments are not necessarily essential to means provided by aspectsof the invention.

FIG. 1 schematically shows the structure of a fabrication apparatus 10for fabricating a porous glass base material for optical fiber accordingto the VAD method. The fabrication apparatus 10 includes a reactionvessel 11, a shaft 12, a core forming burner 41, a core-side-cladforming burner 42 and a surface-side-clad forming burner 43.

The reaction vessel 11 encloses therein the environment in which aporous glass base material is fabricated, in order to prevent the porousglass base material from being contaminated during the fabricationprocess and to prevent the glass fine particles and the like that areproduced during the fabrication process from scattering. In addition,for the purpose of preparing the atmosphere for the formation of theporous glass base material, the reaction vessel 11 has an inlet 13 andan outlet 14.

Through the inlet 13 of the reaction vessel 11, for example, clean airis fed. In this way, clean environment is maintained for fabricating theporous glass base material. Through the outlet 14 of the reaction vessel11, part of the fed air and the glass fine particles that have beensynthesized but not deposited to form the porous glass base material arepassed out of the reaction vessel 11. After passed out of the reactionvessel 11, the glass fine particles are collected outside the reactionvessel 11 and thus prevented from scattering into the surroundingenvironment.

As shown in FIG. 1, the shaft 12 holds the upper end of a seed rod 20using its lower end to hang the seed rod 20 inside the reaction vessel11. In addition, while keeping the seed rod 20 hanging, the shaft 12rotates around the vertical rotation axis and moves up and down togetherwith the seed rod 20. In this way, the seed rod 20, which serves as thetarget on which the glass fine particles are to be deposited is pulledup as a porous glass base material grows thereon, so that a porous glassbase material having a target length can be fabricated.

The core forming burner 41, the core-side-clad forming burner 42, andthe surface-side-clad forming burner 43 each spray, into an oxyhydrogenflame, source material gases such as silicon tetrachloride andoctamethylcyclotetrasiloxane, which are used as the glass sourcematerials, in order to synthesize glass fine particles. The core formingburner 41 is configured to deposit the synthesized glass fine particlesmainly onto the free end of the seed rod 20 and the core-correspondingportion 31 that grows downward from the free end of the seed rod 20.

The core-corresponding portion 31 of the porous glass base material 30is to be eventually formed into a core portion of optical fiber.Germanium tetrachloride or the like is added to the source materialgases fed to the core forming burner 41, as the source material ofgermanium oxide, which serves as the dopants to raise the refractiveindex. Furthermore, the core forming burner 41 receives the delivery ofsilicon tetrachloride serving as the glass source material, a hydrogengas serving as the combustible gas, an oxygen gas serving as thecombustion-supporting gas, a nitrogen gas and an argon gas serving as aseal gas, and the like.

The core-side-clad forming burner 42 is configured to deposit thesynthesized glass fine particles mainly onto the lateral surface of thecore-corresponding portion 31 that has been deposited by the coreforming burner 41. The surface-side-clad forming burner 43 is configuredto further deposit the synthesized glass fine particles mainly onto aportion of the clad-corresponding portion 32 that has been deposited bythe core-side-clad forming burner 42. The formed porous glass basematerial 30 is dehydrated and made transparent in a heating furnace insubsequent steps, to be formed into a glass base material.

The clad-corresponding portion 32 of the porous glass base material 30is eventually formed into a clad portion of optical fiber. Since theclad-corresponding portion 32 is required to have a significantly largervolume than the core-corresponding portion 31, the clad-correspondingportion 32 may be formed by using a plurality of synthesizing burners.In this case, the plurality of synthesizing burners used to form theclad-corresponding portion 32 respectively form different portions ofthe clad-corresponding portion 32.

According to the shown example, the fabrication apparatus 10 is providedwith the core-side-clad forming burner 42 and the surface-side-cladforming burner 43. The core-side-clad forming burner 42 is used to formthe inner portion of the clad-corresponding portion 32 that is adjacentto the core-corresponding portion 31. The surface-side-clad formingburner 43 is positioned adjacent to the core-side-clad forming burner 42and used to form the outer portion defining the surface of theclad-corresponding portion 32. The clad-corresponding portion 32 formedby the core-side-clad forming burner 42 and the clad-correspondingportion 32 formed by the surface-side-clad forming burner 43 are formedinto an integrated clad-corresponding portion 32 in the completed porousglass base material 30.

The core-side-clad forming burner 42 and the surface-side-clad formingburner 43 may receive the delivery of silicon tetrachloride serving asthe glass source material, a hydrogen gas serving as the combustiblegas, an oxygen gas serving as the combustion-supporting gas, an argongas serving as a seal gas, and the like, without addition of dopantsdesigned to change the refractive index. Alternatively, for the purposeof adjusting the refractive index of the clad portion, a germaniumtetrachloride gas, a silicon tetrafluoride gas and the like may be addedto the above-mentioned gases.

The fabrication apparatus 10 further includes a driver 50, a controller60 and a sealing member 70. The driver 50 is arranged outside thereaction vessel 11 and includes an electric goniostage 51 and a burnerholder 52. The burner holder 52 holds the vicinity of the back end ofthe surface-side-clad forming burner 43. The burner holder 52 issupported by the electric goniostage 51. In this way, when the electricgoniostage 51 is operated under the control of the controller 60, thegradient of the surface-side-clad forming burner 43 relative to thehorizontal direction changes.

Here, the gradient θ of the surface-side-clad forming burner 43 ischanged by allowing the surface-side-clad forming burner 43 to rotatearound the virtual rotation axis C defined by the electric goniostage51. As shown in FIG. 1, in the fabrication apparatus 10, the electricgoniostage 51 defines the virtual horizontal rotation axis C for thesurface-side-clad forming burner 43 at the position overlapping the wallsurface of the reaction vessel 11.

In this way, the displacement of the surface-side-clad forming burner 43that accompanies the change in the gradient θ is the smallest at theposition at which the surface-side-clad forming burner 43 intersectswith the wall surface of the reaction vessel 11. This can reduce thesize of the through hole that is formed in the wall surface of thereaction vessel 11 to allow the surface-side-clad forming burner 43 topenetrate through the wall surface of the reaction vessel 11.

As the electric goniostage 51 is used to constitute the driver 50, thecontroller 60 can perform electrical control to change the gradient θ ofthe surface-side-clad forming burner 43 in accordance with the change inthe duration of the deposition or the pulled-up distance. This can makeit easy to automate the control of the gradient of the surface-side-cladforming burner 43, which will be mentioned later.

In the fabrication apparatus 10, the sealing member 70 provides airtightseal between the outside of the wall surface of the reaction vessel 11and the surface of the surface-side-clad forming burner 43. The sealingmember 70 has a tubular shape as a whole, and one of the ends firmlyadhere to the outer surface of the reaction vessel 11 and the other endfirmly adhere to the outer surface of the surface-side-clad formingburner 43.

The sealing member 70 may be formed using a flexible material having ahigh heatproof temperature, for example, silicon rubber. In this way,even if the change in the gradient θ causes a change in the position ofthe surface-side-clad forming burner 43 relative to reaction vessel 11,the inside of the reaction vessel 11 can be disconnected from thesurrounding atmosphere since the sealing member 70 can deform whilemaintaining the airtightness of the reaction vessel 11.

In the fabrication apparatus 10, the virtual rotation axis C of thesurface-side-clad forming burner 43 is positioned so as to lie in thewall surface of the reaction vessel 11. In this way, there is only asmall change in the position of the surface-side-clad forming burner 43relative to the reaction vessel 11, which can reduce the amount of thedeformation of the sealing member 70 caused by the change in gradient θ.This allows the sealing member 70 to achieve improved durability andmakes it easy to maintain the airtightness of the reaction vessel 11.

When the porous glass base material 30 is fabricated using theabove-described fabrication apparatus 10, part of the glass fineparticles synthesized by the core-side-clad forming burner 42 may bedeposited to form the porous glass base material 30 outside the flame ofthe core-side-clad forming burner 42. Such glass fine particles may forma low-density portion. When the shaft 12 is pulled up so that the porousglass base material 30 is moved upward in the drawing, the low-densityportion thus formed is heated by the flame of the surface-side-cladforming burner 43 and resultantly has a higher density.

In the fabrication apparatus 10, the setting conditions such as thegradient and positioning of the core forming burner 41 are determinedbased on the target specifications of the core-corresponding portion 31of the porous glass base material 30 to be fabricated. Since the innerportion of the clad-corresponding portion 32 is directly formed on thesurface of the core-corresponding portion 31, the setting conditions ofthe core-side-clad forming burner 42 also largely depend on the settingconditions of the core forming burner 41 and the like.

On the other hand, the setting conditions of the surface-side-cladforming burner 43 are not particularly limited in any aspects except forthat high deposit efficiency of the glass fine particles is required.Here, the deposit efficiency of the glass fine particles denotes theratio of the glass fine particles deposited to form the porous glassbase material 30 to all the glass fine particles synthesized by thesynthesizing burners.

While the porous glass base material 30 is fabricated using thefabrication apparatus 10, the outer diameter of the porous glass basematerial 30 significantly varies during the initial stage of thedeposition of the glass fine particles. Therefore, in order to keep theabove-described process of forming the porous glass base material 30successfully proceeding, the appropriate setting conditions of thesurface-side-clad forming burner 43 may vary during the fabricationprocess of the porous glass base material 30.

For example, the porous glass base material 30 may crack when thelow-density portion that is formed outside the flame of thecore-side-clad forming burner 42, that is, formed at low temperatures isheated by the flame of the surface-side-clad forming burner 43. Such acrack in the porous glass base material 30 may occur during the initialstage of the deposition of the glass fine particles onto the seed rod 20and is referred to as an initial-stage crack. If an initial-stage crackoccurs, it is required to restart the fabrication process of the porousglass base material 30 from the beginning, which lowers the yield andproductivity.

Even if an initial-stage crack does not occur and the fabricationprocess of the porous glass base material 30 thus successfully proceeds,the outer diameter of the porous glass base material may be unstableduring the initial stage of the fabrication process during which theinitial-stage crack can occur. If there is variation in the outerdiameter of the porous glass base material 30, the ratio of thecore-corresponding portion 31 to the clad-corresponding portion 32 inthe porous glass base material 30 is unstable and the porous glass basematerial 30 cannot be used as an optical fiber base material. Thus, ittakes a lot of time until the fabrication apparatus 10 can successivelyfabricate porous glass base materials 30 that stably have a target outerdiameter, which lowers the productivity of the porous glass basematerials.

To address this issue, the fabrication apparatus 10 is configured to becapable of changing the gradient θ of the surface-side-clad formingburner 43. With such a configuration, until porous glass base materials30 can stably have a target outer diameter, the gradient θ is changed toappropriately adjust the setting conditions of the surface-side-cladforming burner 43 so that the initial-stage crack and the variation inthe outer diameter can be reduced. As shown in FIG. 1, the gradient θ ofthe surface-side-clad forming burner 43 is denoted as the angle of thecentral axis T of the surface-side-clad forming burner 43 with respectto the horizontal plane H.

FIG. 1 also shows the state that can be observed during the relativelyinitial stage of the deposition of the glass fine particles in thefabrication apparatus 10. During this stage, even the largest portion ofthe porous glass base material 30 does not yet have the target outerdiameter. A first timing during the fabrication process of the porousglass base material 30 is set within this period during which the outerdiameter of the porous glass base material 30 has not reached the targetouter diameter.

Here, the first timing is such a timing that the ejection port of thesurface-side-clad forming burner 43 from which the flame is ejectedstarts to face upward relative to the horizontal plane H. The firsttiming may be the timing at which the surface-side-clad forming burner43 starts to deposit the glass fine particle or a timing subsequent tothis timing. Note that, however, the first timing is positioned beforethe gradient θ of the surface-side-clad forming burner 43 is fixed andthe deposition to form the porous glass base material 30 can beperformed under steady conditions.

At the first timing, the surface-side-clad forming burner 43 has such agradient θ that the ejection port of the surface-side-clad formingburner 43 faces downward relative to the horizontal plane H. In otherwords, the gradient θ of the surface-side-clad forming burner 43 is anegative angle relative to the horizontal plane H.

When the core forming burner 41, the core-side-clad forming burner 42and the surface-side-clad forming burner 43 are ignited at the firsttiming to start the synthesis of the glass fine particles and thedeposition of the glass fine particles onto the seed rod 20, thegradient θ of the surface-side-clad forming burner 43 is, for example,−10° at the first timing. In this way, the flame ejected from thesurface-side-clad forming burner 43 becomes continuous with the flamefrom the core-side-clad forming burner 42 on the surface of the porousglass base material 30 that still has a small diameter.

Here, the source material gases may be fed to the core forming burner 41and the core-side-clad forming burner 42 at the same timing, in whichcase the seed rod 20 starts to be pulled up at the same timing. However,the source material gases may be fed to the core forming burner 41 andthe core-side-clad forming burner 42 at different timings. The sourcematerial gases may be fed to the surface-side-clad forming burner 43 sothat the surface-side-clad forming burner 43 can start the deposition ofthe glass fine particles at a later timing than the timing for the otherburners, for example, approximately one hour later. Therefore, theabove-mentioned first timing may be determined based on the timing atwhich the surface-side-clad forming burner 43 is to be ignited.

During the initial stage of the deposition of the glass fine particlesin the fabrication apparatus 10, the amounts of the gases ejected fromthe respective burners may be reduced since the glass fine particles aredeposited onto a thin seed rod 20. The amounts of the gases ejected maybe gradually increased as the duration of the deposition elapses or thedistance by which the shaft 12 pulls up the seed rod 20 increases or thelike, during the period lasting until the outer diameter of the porousglass base material 30 being formed reaches the target outer diameter sothat the glass fine particles can be deposited under steady conditions.

FIG. 2 shows a different stage during the fabrication process of theporous glass base material 30 using the fabrication apparatus 10. Theporous glass base material 30 being formed has significantly growncompared with the state shown in FIG. 1 but the outer diameter of theporous glass base material 30 has not yet reached the target outerdiameter. Compared with the state shown in FIG. 1, the porous glass basematerial 30 has been pulled up to a higher position through the seed rod20 and the shaft 12 of the fabrication apparatus 10.

Furthermore, when the fabrication apparatus 10 is in the state shown inFIG. 2, as the surface-side-clad forming burner 43 has been driven bythe driver 50, the ejection port of the surface-side-clad forming burner43 has rotated to be generally coplanar with the horizontal plane H sothat the gradient θ is approximately 0. At this stage, the intersectionangle between the direction in which the flame is ejected from thecore-side-clad forming burner 42 and the direction in which the flame isejected from the surface-side-clad forming burner 43 is relativelysmall. Since the porous glass base material 30 has grown to have a largeouter diameter, however, the flame of the core-side-clad forming burner42 is continuous with the flame of the surface-side-clad forming burner43 on the surface of the porous glass base material 30 and there is thusno gap between the flames.

FIG. 3 shows a further subsequent stage during the fabrication processof the porous glass base material 30 using the fabrication apparatus 10.Compared with the state shown in FIG. 2, the porous glass base material30 has been further pulled up by the shaft 12 of the fabricationapparatus 10. Also, the outer diameter of the porous glass base material30 being formed has reached the target outer diameter. This means thatan effective portion that can be used as optical fiber base material hasalready started to be formed. The effective portion is a continuousportion having a constant target outer diameter.

When the fabrication apparatus 10 is in the state shown in FIG. 3, thedeposition of the glass fine particles by the surface-side-clad formingburner 43 starts to form the effective portion of the porous glass basematerial 30 that continuously have a constant target outer diameter.Accordingly, the fabrication apparatus 10 is in the steady state, inwhich the fabrication apparatus 10 can grow the porous glass basematerial 30 having a constant outer diameter in the length direction.This means that a second timing during the fabrication process of theporous glass base material 30 using the fabrication apparatus 10 hasbeen already passed.

Here, the second timing is later than the first timing and determined inadvance in accordance with the target outer diameter of the porous glassbase material 30 to be fabricated. For example, varying the angle of thesurface-side-clad forming burner 43 is stopped 200 minutes after thestart, after which the conditions such as the source material gases fedto the surface-side-clad forming burner 43 are kept unchanged. Inaddition, the rate at which the seed rod 20 is pulled up is also keptconstant. For the reasons stated above, the second timing may be definedas the timing at which the conditions for the formation of the porousglass base material 30 being fabricated become constant.

At the second timing, changing the orientation of the ejection port ofthe surface-side-clad forming burner 43 further upward, that is,increasing the gradient θ relative to the horizontal plane H in thepositive direction is stopped and the gradient θ of thesurface-side-clad forming burner 43 is fixed. In other words, in thefabrication apparatus 10, during the period from the above-describedfirst timing to the above-described second timing, the orientation ofthe ejection port of the surface-side-clad forming burner 43 is changedfurther upward, that is, the gradient θ relative to the horizontal planeH is increased in the positive direction while the glass fine particlesare deposited.

Here, the gradient θ of the surface-side-clad forming burner 43 may bechanged in a continuous or stepwise manner. In particular, when astepping motor is used as the driver, the gradient θ is unavoidablychanged in a stepwise manner. If the change occurs in sufficiently smallsteps, however, there are hardly differences between the stepwisechanges and continuous changes. When the change occurs in excessivelylarge steps, on the other hand, the surface of the porous glass basematerial 30 being fabricated experiences a sudden change in temperature,which may cause cracks and the like.

Since the state shown in FIG. 3 can be observed after the second timinghas been passed, the gradient θ of the surface-side-clad forming burner43 is fixed to a value realizing the orientation corresponding to thesteady state. In the shown example, the orientation of thesurface-side-clad forming burner 43 is slightly upward. In this case,the direction in which the flame is ejected from the core-side-cladforming burner 42 is further moved away from the direction in which theflame is ejected from the surface-side-clad forming burner 43. Since theouter diameter of the porous glass base material 30 has grown to besufficiently large, however, the flame of the core-side-clad formingburner 42 is continuous with the flame of the surface-side-clad formingburner 43 on the surface of the porous glass base material 30 so thatthere is no gap between the flames.

The gradient θ of the surface-side-clad forming burner 43 usedsubsequent to the second timing is determined in advance based, forexample, on the target outer diameter of the porous glass base material30. Here, even after changing the gradient θ of the surface-side-cladforming burner 43 is stopped, the synthesis of the glass fine particlesby the core forming burner 41, the core-side-clad forming burner 42 andthe surface-side-clad forming burner 43 continues so that the porousglass base material 30 continues growing in the longitudinal direction.In this way, the porous glass base material 30 continues to be formeduntil the effective portion of the porous glass base material 30 thathas a constant outer diameter reaches a target length.

According to the example shown in FIGS. 1 to 3, the fabricationapparatus 10 keeps the orientation of the surface-side-clad formingburner 43 downward during the initial stage of the fabrication processof the porous glass base material 30 so as to reduce the gap between theflame of the core-side-clad forming burner 42 and the flame of thesurface-side-clad forming burner 43 on the surface of the porous glassbase material 30 and avoid a gap from being formed between the flames.This can reduce the formation of the low-density portion in the porousglass base material 30, thereby preventing the initial-stage crack fromoccurring.

The fabrication apparatus 10 forms the porous glass base material 30while keeping the flame of the core-side-clad forming burner 42continuous with the flame of the surface-side-clad forming burner 43 onthe surface of the porous glass base material 30. In this way, thelow-density portion, which may possibly be formed in the porous glassbase material 30 by the core-side-clad forming burner 42, can beimmediately heated by the surface-side-clad forming burner 43 and canaccomplish a high density. Consequently, the formation of thelow-density portion can be further reduced.

In other words, when the porous glass base material 30 is fabricatedusing the fabrication apparatus 10, the gradient θ of thesurface-side-clad forming burner 43 is preferably changed, as theformation of the porous glass base material 30 proceeds, within such arange that the flame ejected from the surface-side-clad forming burner43 is always kept continuous with the flame ejected from thecore-side-clad forming burner 42 on the surface of the porous glass basematerial 30. This can prevent the initial-stage crack from occurring inthe porous glass base material 30 being formed.

In combination with controlling the gradient θ of the surface-side-cladforming burner 43 in the above-described manner, the amounts of thegases ejected from the respective burners may be regulated for thepurposes of reducing the initial-stage crack. Specifically speaking, inorder to reduce the initial-stage crack, the amounts of the gasesejected from the respective burners may be set low during the initialstage of the deposition of the glass fine particles and increased at acontrolled rate until the outer diameter of the porous glass basematerial 30 being formed reaches the target outer diameter so that theglass fine particles can be deposited under steady conditions.

FIG. 4 is used to describe the shape of the porous glass base materialfabricated using the fabrication apparatus 10. FIG. 4 also compares theshapes of the porous glass base materials 30 fabricated in two differentmethods using the fabrication apparatus 10.

The porous glass base material 30 shown in the upper section (a) has anon-effective portion 81 that is shown on the left side in the drawingand has a gradually increasing outer diameter, an effective portion 82that has a substantially constant outer diameter and a non-effectiveportion 83 that is shown on the right side in the drawing and has agradually decreasing outer diameter. Here, the non-effective portion 81is formed during the initial stage of the fabrication process of theporous glass base material 30 using the fabrication apparatus 10 and thenon-effective portion 83 is formed during the terminal stage of thefabrication process of the porous glass base material 30.

In the non-effective portions 81 and 83, the ratio of thecore-corresponding portion 31 to the clad-corresponding portion 32varies among the cross-sections orthogonal to the longitudinaldirection. Consequently, even if the non-effective portions 81 and 83are processed into transparent glass and drawn, the non-effectiveportions 81 and 83 cannot be fabricated into optical fiber. On the otherhand, the effective portion 82 has a constant target outer diameter D₀across its entire length E. Therefore, the ratio of thecore-corresponding portion 31 to the clad-corresponding portion 32 isconstant in every cross-section orthogonal to the longitudinaldirection. Consequently, the effective portion 82 can be used as anoptical fiber base material that can be fabricated into optical fiber bydrawing.

While the porous glass base material 30 shown in the upper section (a)in the drawing is formed by depositing the glass fine particles, thegradient θ of the surface-side-clad forming burner 43 is changed from anegative angle to a positive angle relative to the horizontal plane H.Accordingly, the gradient θ of the surface-side-clad forming burner 43is a negative angle relative to the horizontal plane H during theinitial stage of the deposition of the glass fine particles. This meansthat the flame of the core-side-clad forming burner 42 is close to theflame of the surface-side-clad forming burner 43. Consequently, at theportion to which the flames are applied, the deposition of the glassfine particles is carried out at a high rate.

In this manner, the outer diameter of the porous glass base material 30can swiftly reach the target outer diameter. In the example shown, atthe position substantially the same as the position at which thecore-corresponding portion 31 starts to be formed on the free end of theseed rod 20, the clad-corresponding portion 32 reaches the target outerdiameter D₀ and the effective portion 82 of the porous glass basematerial 30 starts to be formed.

The porous glass base material 30 shown in the lower section (b) of thedrawing also has a non-effective portion 81 that is formed during theinitial stage of the fabrication process, an effective portion 82 formedafter the non-effective portion 81, and a non-effective portion 83 thatis formed during the terminal stage of the fabrication process. Thisporous glass base material 30 is formed in such a manner that thegradient θ of the surface-side-clad forming burner 43 is fixed fromstart to finish during the deposition of the glass fine particles. Here,the fixed value of the gradient θ of the surface-side-clad formingburner 43 is optimized for the formation of the effective portion 82,which is carried out after the outer diameter reaches the target outerdiameter D₀.

Accordingly, during the formation of the non-effective portion 81, thereis a gap between the flame of the core-side-clad forming burner 42 andthe flame of the surface-side-clad forming burner 43. For this reason,the glass fine particles are deposited at a low rate, it takes a lot oftime until the outer diameter of the porous glass base material 30reaches the target outer diameter D₀, and the length A₂ of the initialnon-effective portion 81 in the fabricated porous glass base material 30is resultantly longer than the length A₁ of the non-effective portion 81in the porous glass base material 30 shown in the upper section.

As described above, if the gradient θ of the surface-side-clad formingburner 43 is fixed, it requires a longer time to finally fabricate theporous glass base material 30 having the effective portion 82 of thesame length E. Furthermore, it is required to increase the amounts ofthe materials and fuels to fabricate the porous glass base material 30,and more glass fine particles are not deposited to form the porous glassbase material 30 and remain within the reaction vessel 11. Such excessglass fine particles may adhere to the inner surface of the reactionvessel 11 and then come off and fall in the agglomerated form to adhereonto the porous glass base material 30. This may increase theprobability of foams in the optical fiber glass base materials.

Experimental Example 1

The fabrication apparatus 10 was used to fabricate a porous glass basematerial 30 having an effective portion with a length E of 1400 mm and atarget outer diameter of 250 mm. The gradient θ of the surface-side-cladforming burner 43 was defined to have a positive value when the ejectionport of the surface-side-clad forming burner 43 faced upward relative tothe horizontal plane H. At the first timing at which the synthesis ofthe glass fine particles by the surface-side-clad forming burner 43started, the gradient θ was set to −10°, that is, the ejection port ofthe surface-side-clad forming burner 43 was controlled to face downward.

In the fabrication apparatus 10, the controller 60 controlled the driver50 in such a manner that the gradient θ of the surface-side-clad formingburner 43 reached +6° at the second timing by which the shaft 12 hadbeen pulled up by 200 mm. Accordingly, during the period from the statethat is shown in FIG. 1 and observed at the first timing to the statethat is shown in FIG. 3 and observed at the second timing, the gradientθ of the surface-side-clad forming burner 43 was continuously changed atthe rate of 0.08°/mm with respect to the amount by which the shaft 12was pulled up.

The porous glass base material 30 obtained in the above-described mannerhad an entire length L₁ of 1550 mm, including non-effective portions 81and 83. This means that the ratio of the length E of the effectiveportion to the entire length L₁ of the porous glass base material 30 was90.3%.

Under the same settings, 50 porous glass base materials 30 weresuccessively fabricated but none of them had an initial-stage crack. Theobtained porous glass base materials 30 were dehydrated and processedinto transparent glass in a heating furnace. The resulting 50 porousglass base materials 30 had 0.04 foams on average.

Comparative Example 1

For the purpose of comparison, 50 porous glass base materials 30 werefabricated that had an effective portion with a length E of 1400 mm anda target outer diameter of 250 mm with the gradient θ of thesurface-side-clad forming burner 43 being fixed at +6° from start tofinish. The porous glass base materials 30 had an entire length L₂ of1700 mm including non-effective portions. Consequently, the ratio of thelength E of the effective portion to the entire length L₂ of the porousglass base material 30 was 82.4%.

While the 50 porous glass base materials 30 were fabricated, aninitial-stage crack occurred three times. The obtained porous glass basematerials 30 were dehydrated and processed into transparent glass in aheating furnace. The resulting 50 porous glass base materials 30 had0.12 foams on average.

As described above, the fabrication apparatus 10 including thesurface-side-clad forming burner 43 that has variable orientation wasused, and the orientation of the surface-side-clad forming burner 43 waschanged during a certain period before the outer diameter of the porousglass base material 30 being formed reached the target outer diameter.In this manner, the outer diameter of the porous glass base material 30could swiftly reach the target outer diameter and the effective portioncould occupy a higher ratio in the resulting porous glass base material30. As a consequence, the yield per unit of source materials can beimproved and, at the same time, the productivity of the optical fiberglass base material can be improved since the period of time required tocomplete the fabrication is shortened.

If the period of time required to complete the fabrication of the porousglass base material 30 is shortened, the amount of the agglomerated sootcan be also reduced. Such agglomerated soot may be formed in such amanner that the glass fine particles adhere to the inner surface of thereaction vessel 11 since they are not deposited to form the porous glassbase material 30 in any of the steps of the fabrication process. Thisleads to reduction in the foams that are produced in the glass basematerial by the soot that adhere to the porous glass base material 30.

During the initial stage of the deposition to form theclad-corresponding portion 32, which comes before the outer diameter ofthe porous glass base material 30 reaches the target outer diameter, theorientation of the surface-side-clad forming burner 43 is controlled tobe downward, compared with the orientation employed during the steadystate which comes after the outer diameter of the porous glass basematerial 30 reaches the target outer diameter. In this manner, aninitial-stage crack can be also reduced during the fabrication processof the porous glass base material 30. As a result, the porous glass basematerial 30 can achieve the improved yield and the cost of fabricatingthe optical fiber glass base material can be reduced.

Here, the fabrication apparatus 10 may have three or more clad formingburners. When there are three or more clad forming burners, thesynthesizing burner that is designed to form the outermost surface ofthe clad-corresponding portion 32 may be labeled as thesurface-side-clad forming burner 43.

While the embodiments of the present invention have been described, thetechnical scope of the invention is not limited to the above describedembodiments. It is apparent to persons skilled in the art that variousalterations and improvements can be added to the above-describedembodiments. It is also apparent from the scope of the claims that theembodiments added with such alterations or improvements can be includedin the technical scope of the invention.

The operations, procedures, steps, and stages of each process performedby an apparatus, system, program, and method shown in the claims,embodiments, or diagrams can be performed in any order as long as theorder is not indicated by “prior to,” “before,” or the like and as longas the output from a previous process is not used in a later process.Even if the process flow is described using phrases such as “first” or“next” in the claims, embodiments, or diagrams, it does not necessarilymean that the process must be performed in this order.

EXPLANATION OF REFERENCES

10: fabrication apparatus, 11: reaction vessel, 12: shaft, 13: inlet,14: outlet, 20: seed rod, 30: porous glass base material, 31:core-corresponding portion, 32: clad-corresponding portion, 41: coreforming burner, 42: core-side-clad forming burner, 43: surface-side-cladforming burner, 50: driver, 51: electric goniostage, 52: burner, 60:controller, 70: sealing member, 81 and 83: non-effective portion, 82:effective portion

What is claimed is:
 1. A fabrication method for fabricating a porousglass base material for optical fiber, in which a core-correspondingportion corresponding to a core of optical fiber is formed by depositingglass fine particles onto a hanging seed rod, and at least a portion ofa clad-corresponding portion corresponding to a clad of the opticalfiber is formed by depositing glass fine particles onto thecore-corresponding portion, wherein the fabrication method includes aperiod during which, while glass fine particles are being deposited, agradient of a clad forming burner used to form an outermost layer of theclad-corresponding portion is changed toward a predetermined gradientrelative to the porous glass base material for optical fiber from adownward gradient compared with the predetermined gradient.
 2. Thefabrication method for fabricating a porous glass base material foroptical fiber as set forth in claim 1, wherein at a start of thedeposition of the glass fine particles, the clad forming burner facesdownward relative to a horizontal plane.
 3. The fabrication method forfabricating a porous glass base material for optical fiber as set forthin claim 2, wherein the predetermined gradient is an upward gradient. 4.The fabrication method for fabricating a porous glass base material foroptical fiber as set forth in claim 3, wherein the gradient of the cladforming burner goes beyond the horizontal plane before an outer diameterof the porous glass base material reaches a target outer diameter of theporous glass base material.
 5. The fabrication method for fabricating aporous glass base material for optical fiber as set forth in claim 1,wherein the predetermined gradient is determined in advance based on atarget outer diameter of the porous glass base material for opticalfiber to be fabricated.
 6. The fabrication method for fabricating aporous glass base material for optical fiber as set forth in claim 1,wherein an orientation of the clad forming burner is changed within sucha range that a flame ejected from the clad forming burner is continuouswith a flame of a different forming burner that is adjacent to the cladforming burner.